Make PipelineSelection a proper enum type

PipelineSelection is one of the few stringy types that weren't
converted to FastEnum, presumably because we lacked a mechanism for
only allowing a sub-set of options as CLI arguments.

We've re-designed this since, and as part of the UI/UX refactor we'd
like to generally clean this, but that is probably still a while out.

Since that hasn't happened, for now, this adds a feature to the
FastEnumType that allows specifying only a subset of values is allowed
for a specific command, so that we can use the type as a proper
enum.

We also get rid of a number of accidental uses of strings, and move
PipelineSelection to buildstream.types so that we don't have a
significant import overhead for it.

5 files changed, 153 insertions, 110 deletions

diff --git a/src/buildstream/_context.py b/src/buildstream/_context.py
index 47001440a..9642edd49 100644
--- a/src/buildstream/_context.py
+++ b/src/buildstream/_context.py
@@ -29,7 +29,7 @@ from ._platform import Platform
 from ._artifactcache import ArtifactCache
 from ._sourcecache import SourceCache
 from ._cas import CASCache, CASLogLevel
-from .types import _CacheBuildTrees, _SchedulerErrorAction
+from .types import _CacheBuildTrees, _PipelineSelection, _SchedulerErrorAction
 from ._workspaces import Workspaces, WorkspaceProjectCache
 from .node import Node
 from .sandbox import SandboxRemote
@@ -361,13 +361,14 @@ class Context:
         build.validate_keys(["max-jobs", "dependencies"])
         self.build_max_jobs = build.get_int("max-jobs")
 
-        self.build_dependencies = build.get_str("dependencies")
-        if self.build_dependencies not in ["plan", "all"]:
+        dependencies = build.get_str("dependencies")
+        if dependencies not in ["plan", "all"]:
             provenance = build.get_scalar("dependencies").get_provenance()
             raise LoadError(
                 "{}: Invalid value for 'dependencies'. Choose 'plan' or 'all'.".format(provenance),
                 LoadErrorReason.INVALID_DATA,
             )
+        self.build_dependencies = _PipelineSelection(dependencies)
 
         # Load per-projects overrides
         self._project_overrides = defaults.get_mapping("projects", default={})
diff --git a/src/buildstream/_frontend/cli.py b/src/buildstream/_frontend/cli.py
index 051f5d7f7..365f5c421 100644
--- a/src/buildstream/_frontend/cli.py
+++ b/src/buildstream/_frontend/cli.py
@@ -9,7 +9,7 @@ from .. import _yaml
 from .._exceptions import BstError, LoadError, AppError
 from .._versions import BST_FORMAT_VERSION
 from .complete import main_bashcomplete, complete_path, CompleteUnhandled
-from ..types import _CacheBuildTrees, _SchedulerErrorAction
+from ..types import _CacheBuildTrees, _SchedulerErrorAction, _PipelineSelection
 from ..utils import _get_compression, UtilError
 
 
@@ -19,11 +19,22 @@ from ..utils import _get_compression, UtilError
 
 
 class FastEnumType(click.Choice):
-    def __init__(self, enum):
+    def __init__(self, enum, options=None):
         self._enum = enum
-        super().__init__(enum.values())
+
+        if options is None:
+            options = enum.values()
+        else:
+            options = [option.value for option in options]
+
+        super().__init__(options)
 
     def convert(self, value, param, ctx):
+        # This allows specifying default values as instances of the
+        # enum
+        if isinstance(value, self._enum):
+            value = value.value
+
         return self._enum(super().convert(value, param, ctx))
 
 
@@ -438,7 +449,13 @@ def init(app, project_name, format_version, element_path, force, target_director
 #                          Build Command                         #
 ##################################################################
 @cli.command(short_help="Build elements in a pipeline")
-@click.option("--deps", "-d", default=None, type=click.Choice(["plan", "all"]), help="The dependencies to build")
+@click.option(
+    "--deps",
+    "-d",
+    default=None,
+    type=FastEnumType(_PipelineSelection, [_PipelineSelection.PLAN, _PipelineSelection.ALL]),
+    help="The dependencies to build",
+)
 @click.option(
     "--remote", "-r", default=None, help="The URL of the remote cache (defaults to the first configured cache)"
 )
@@ -484,9 +501,18 @@ def build(app, elements, deps, remote):
 @click.option(
     "--deps",
     "-d",
-    default="all",
+    default=_PipelineSelection.ALL,
     show_default=True,
-    type=click.Choice(["none", "plan", "run", "build", "all"]),
+    type=FastEnumType(
+        _PipelineSelection,
+        [
+            _PipelineSelection.NONE,
+            _PipelineSelection.PLAN,
+            _PipelineSelection.RUN,
+            _PipelineSelection.BUILD,
+            _PipelineSelection.ALL,
+        ],
+    ),
     help="The dependencies to show",
 )
 @click.option(
@@ -641,12 +667,11 @@ def shell(app, element, sysroot, mount, isolate, build_, cli_buildtree, pull_, c
     """
     from ..element import Scope
     from .._project import HostMount
-    from .._pipeline import PipelineSelection
 
     scope = Scope.BUILD if build_ else Scope.RUN
 
     # We may need to fetch dependency artifacts if we're pulling the artifact
-    selection = PipelineSelection.ALL if pull_ else PipelineSelection.NONE
+    selection = _PipelineSelection.ALL if pull_ else _PipelineSelection.NONE
     use_buildtree = None
 
     with app.initialized():
@@ -750,9 +775,9 @@ def source():
 @click.option(
     "--deps",
     "-d",
-    default="plan",
+    default=_PipelineSelection.PLAN,
     show_default=True,
-    type=click.Choice(["none", "plan", "all"]),
+    type=FastEnumType(_PipelineSelection, [_PipelineSelection.NONE, _PipelineSelection.PLAN, _PipelineSelection.ALL]),
     help="The dependencies to fetch",
 )
 @click.option(
@@ -803,9 +828,9 @@ def source_fetch(app, elements, deps, except_, remote):
 @click.option(
     "--deps",
     "-d",
-    default="none",
+    default=_PipelineSelection.NONE,
     show_default=True,
-    type=click.Choice(["none", "all"]),
+    type=FastEnumType(_PipelineSelection, [_PipelineSelection.NONE, _PipelineSelection.ALL]),
     help="The dependencies to track",
 )
 @click.option("--cross-junctions", "-J", is_flag=True, help="Allow crossing junction boundaries")
@@ -839,8 +864,8 @@ def source_track(app, elements, deps, except_, cross_junctions):
 
         # Substitute 'none' for 'redirect' so that element redirections
         # will be done
-        if deps == "none":
-            deps = "redirect"
+        if deps == _PipelineSelection.NONE:
+            deps = _PipelineSelection.REDIRECT
         app.stream.track(elements, selection=deps, except_targets=except_, cross_junctions=cross_junctions)
 
 
@@ -855,9 +880,12 @@ def source_track(app, elements, deps, except_, cross_junctions):
 @click.option(
     "--deps",
     "-d",
-    default="none",
+    default=_PipelineSelection.NONE,
     show_default=True,
-    type=click.Choice(["build", "none", "run", "all"]),
+    type=FastEnumType(
+        _PipelineSelection,
+        [_PipelineSelection.BUILD, _PipelineSelection.NONE, _PipelineSelection.RUN, _PipelineSelection.ALL],
+    ),
     help="The dependencies whose sources to checkout",
 )
 @click.option(
@@ -1097,9 +1125,12 @@ def artifact():
 @click.option(
     "--deps",
     "-d",
-    default="none",
+    default=_PipelineSelection.NONE,
     show_default=True,
-    type=click.Choice(["build", "run", "all", "none"]),
+    type=FastEnumType(
+        _PipelineSelection,
+        [_PipelineSelection.BUILD, _PipelineSelection.RUN, _PipelineSelection.ALL, _PipelineSelection.NONE],
+    ),
     help="The dependencies we also want to show",
 )
 @click.argument("artifacts", type=click.Path(), nargs=-1)
@@ -1120,9 +1151,12 @@ def artifact_show(app, deps, artifacts):
 @click.option(
     "--deps",
     "-d",
-    default="run",
+    default=_PipelineSelection.RUN,
     show_default=True,
-    type=click.Choice(["run", "build", "none", "all"]),
+    type=FastEnumType(
+        _PipelineSelection,
+        [_PipelineSelection.RUN, _PipelineSelection.BUILD, _PipelineSelection.NONE, _PipelineSelection.ALL],
+    ),
     help="The dependencies to checkout",
 )
 @click.option("--integrate/--no-integrate", default=None, is_flag=True, help="Whether to run integration commands")
@@ -1216,9 +1250,9 @@ def artifact_checkout(app, force, deps, integrate, hardlinks, tar, compression,
 @click.option(
     "--deps",
     "-d",
-    default="none",
+    default=_PipelineSelection.NONE,
     show_default=True,
-    type=click.Choice(["none", "all"]),
+    type=FastEnumType(_PipelineSelection, [_PipelineSelection.NONE, _PipelineSelection.ALL]),
     help="The dependency artifacts to pull",
 )
 @click.option(
@@ -1265,9 +1299,9 @@ def artifact_pull(app, artifacts, deps, remote):
 @click.option(
     "--deps",
     "-d",
-    default="none",
+    default=_PipelineSelection.NONE,
     show_default=True,
-    type=click.Choice(["none", "all"]),
+    type=FastEnumType(_PipelineSelection, [_PipelineSelection.NONE, _PipelineSelection.ALL]),
     help="The dependencies to push",
 )
 @click.option(
@@ -1389,9 +1423,12 @@ def artifact_list_contents(app, artifacts, long_):
 @click.option(
     "--deps",
     "-d",
-    default="none",
+    default=_PipelineSelection.NONE,
     show_default=True,
-    type=click.Choice(["none", "run", "build", "all"]),
+    type=FastEnumType(
+        _PipelineSelection,
+        [_PipelineSelection.NONE, _PipelineSelection.RUN, _PipelineSelection.BUILD, _PipelineSelection.ALL],
+    ),
     help="The dependencies to delete",
 )
 @click.argument("artifacts", type=click.Path(), nargs=-1)
@@ -1425,9 +1462,9 @@ def artifact_delete(app, artifacts, deps):
 @click.option(
     "--deps",
     "-d",
-    default="plan",
+    default=_PipelineSelection.PLAN,
     show_default=True,
-    type=click.Choice(["none", "plan", "all"]),
+    type=FastEnumType(_PipelineSelection, [_PipelineSelection.NONE, _PipelineSelection.PLAN, _PipelineSelection.ALL]),
     help="The dependencies to fetch",
 )
 @click.argument("elements", nargs=-1, type=click.Path(readable=False))
@@ -1451,9 +1488,9 @@ def fetch(app, elements, deps, except_):
 @click.option(
     "--deps",
     "-d",
-    default="none",
+    default=_PipelineSelection.NONE,
     show_default=True,
-    type=click.Choice(["none", "all"]),
+    type=FastEnumType(_PipelineSelection, [_PipelineSelection.NONE, _PipelineSelection.ALL]),
     help="The dependencies to track",
 )
 @click.option("--cross-junctions", "-J", is_flag=True, help="Allow crossing junction boundaries")
@@ -1472,9 +1509,9 @@ def track(app, elements, deps, except_, cross_junctions):
 @click.option(
     "--deps",
     "-d",
-    default="run",
+    default=_PipelineSelection.RUN,
     show_default=True,
-    type=click.Choice(["run", "build", "none"]),
+    type=FastEnumType(_PipelineSelection, [_PipelineSelection.RUN, _PipelineSelection.BUILD, _PipelineSelection.NONE]),
     help="The dependencies to checkout",
 )
 @click.option("--integrate/--no-integrate", default=True, help="Run integration commands (default is to run commands)")
@@ -1505,9 +1542,9 @@ def checkout(app, element, location, force, deps, integrate, hardlinks, tar):
 @click.option(
     "--deps",
     "-d",
-    default="none",
+    default=_PipelineSelection.NONE,
     show_default=True,
-    type=click.Choice(["none", "all"]),
+    type=FastEnumType(_PipelineSelection, [_PipelineSelection.NONE, _PipelineSelection.ALL]),
     help="The dependency artifacts to pull",
 )
 @click.option("--remote", "-r", help="The URL of the remote cache (defaults to the first configured cache)")
@@ -1525,9 +1562,9 @@ def pull(app, elements, deps, remote):
 @click.option(
     "--deps",
     "-d",
-    default="none",
+    default=_PipelineSelection.NONE,
     show_default=True,
-    type=click.Choice(["none", "all"]),
+    type=FastEnumType(_PipelineSelection, [_PipelineSelection.NONE, _PipelineSelection.ALL]),
     help="The dependencies to push",
 )
 @click.option(
diff --git a/src/buildstream/_pipeline.py b/src/buildstream/_pipeline.py
index aee0c55da..8f356e9b1 100644
--- a/src/buildstream/_pipeline.py
+++ b/src/buildstream/_pipeline.py
@@ -31,36 +31,7 @@ from ._message import Message, MessageType
 from ._profile import Topics, PROFILER
 from . import Scope, Consistency
 from ._project import ProjectRefStorage
-
-
-# PipelineSelection()
-#
-# Defines the kind of pipeline selection to make when the pipeline
-# is provided a list of targets, for whichever purpose.
-#
-# These values correspond to the CLI `--deps` arguments for convenience.
-#
-class PipelineSelection:
-
-    # Select only the target elements in the associated targets
-    NONE = "none"
-
-    # As NONE, but redirect elements that are capable of it
-    REDIRECT = "redirect"
-
-    # Select elements which must be built for the associated targets to be built
-    PLAN = "plan"
-
-    # All dependencies of all targets, including the targets
-    ALL = "all"
-
-    # All direct build dependencies and their recursive runtime dependencies,
-    # excluding the targets
-    BUILD = "build"
-
-    # All direct runtime dependencies and their recursive runtime dependencies,
-    # including the targets
-    RUN = "run"
+from .types import _PipelineSelection
 
 
 # Pipeline()
@@ -219,7 +190,7 @@ class Pipeline:
     #
     # Args:
     #    targets (list of Element): The target Elements
-    #    mode (PipelineSelection): The PipelineSelection mode
+    #    mode (_PipelineSelection): The PipelineSelection mode
     #
     # Various commands define a --deps option to specify what elements to
     # use in the result, this function reports a list that is appropriate for
@@ -227,10 +198,9 @@ class Pipeline:
     #
     def get_selection(self, targets, mode, *, silent=True):
 
-        elements = None
-        if mode == PipelineSelection.NONE:
+        if mode == _PipelineSelection.NONE:
             elements = targets
-        elif mode == PipelineSelection.REDIRECT:
+        elif mode == _PipelineSelection.REDIRECT:
             # Redirect and log if permitted
             elements = []
             for t in targets:
@@ -239,14 +209,14 @@ class Pipeline:
                     self._message(MessageType.INFO, "Element '{}' redirected to '{}'".format(t.name, new_elm.name))
                 if new_elm not in elements:
                     elements.append(new_elm)
-        elif mode == PipelineSelection.PLAN:
+        elif mode == _PipelineSelection.PLAN:
             elements = self.plan(targets)
         else:
-            if mode == PipelineSelection.ALL:
+            if mode == _PipelineSelection.ALL:
                 scope = Scope.ALL
-            elif mode == PipelineSelection.BUILD:
+            elif mode == _PipelineSelection.BUILD:
                 scope = Scope.BUILD
-            elif mode == PipelineSelection.RUN:
+            elif mode == _PipelineSelection.RUN:
                 scope = Scope.RUN
 
             elements = list(self.dependencies(targets, scope))
diff --git a/src/buildstream/_stream.py b/src/buildstream/_stream.py
index c2945a2f6..db9794c45 100644
--- a/src/buildstream/_stream.py
+++ b/src/buildstream/_stream.py
@@ -48,10 +48,10 @@ from ._scheduler import (
     JobStatus,
 )
 from .element import Element
-from ._pipeline import Pipeline, PipelineSelection
+from ._pipeline import Pipeline
 from ._profile import Topics, PROFILER
 from ._state import State
-from .types import _KeyStrength, _SchedulerErrorAction
+from .types import _KeyStrength, _PipelineSelection, _SchedulerErrorAction
 from .plugin import Plugin
 from . import utils, _yaml, _site
 from . import Scope
@@ -143,7 +143,7 @@ class Stream:
     #
     # Args:
     #    targets (list of str): Targets to pull
-    #    selection (PipelineSelection): The selection mode for the specified targets
+    #    selection (_PipelineSelection): The selection mode for the specified targets
     #    except_targets (list of str): Specified targets to except from fetching
     #    use_artifact_config (bool): If artifact remote configs should be loaded
     #
@@ -153,7 +153,7 @@ class Stream:
         self,
         targets,
         *,
-        selection=PipelineSelection.NONE,
+        selection=_PipelineSelection.NONE,
         except_targets=(),
         use_artifact_config=False,
         load_refs=False
@@ -267,14 +267,14 @@ class Stream:
     #
     # Args:
     #    targets (list of str): Targets to build
-    #    selection (PipelineSelection): The selection mode for the specified targets
+    #    selection (_PipelineSelection): The selection mode for the specified targets
     #    ignore_junction_targets (bool): Whether junction targets should be filtered out
     #    remote (str): The URL of a specific remote server to push to, or None
     #
     # If `remote` specified as None, then regular configuration will be used
     # to determine where to push artifacts to.
     #
-    def build(self, targets, *, selection=PipelineSelection.PLAN, ignore_junction_targets=False, remote=None):
+    def build(self, targets, *, selection=_PipelineSelection.PLAN, ignore_junction_targets=False, remote=None):
 
         use_config = True
         if remote:
@@ -300,7 +300,7 @@ class Stream:
 
             # fetch blobs of targets if options set
             if self._context.pull_artifact_files:
-                scope = Scope.ALL if selection == PipelineSelection.ALL else Scope.RUN
+                scope = Scope.ALL if selection == _PipelineSelection.ALL else Scope.RUN
                 for element in self.targets:
                     element._set_artifact_files_required(scope=scope)
 
@@ -331,11 +331,11 @@ class Stream:
     #
     # Args:
     #    targets (list of str): Targets to fetch
-    #    selection (PipelineSelection): The selection mode for the specified targets
+    #    selection (_PipelineSelection): The selection mode for the specified targets
     #    except_targets (list of str): Specified targets to except from fetching
     #    remote (str|None): The URL of a specific remote server to pull from.
     #
-    def fetch(self, targets, *, selection=PipelineSelection.PLAN, except_targets=None, remote=None):
+    def fetch(self, targets, *, selection=_PipelineSelection.PLAN, except_targets=None, remote=None):
 
         use_source_config = True
         if remote:
@@ -358,14 +358,14 @@ class Stream:
     #
     # Args:
     #    targets (list of str): Targets to track
-    #    selection (PipelineSelection): The selection mode for the specified targets
+    #    selection (_PipelineSelection): The selection mode for the specified targets
     #    except_targets (list of str): Specified targets to except from tracking
     #    cross_junctions (bool): Whether tracking should cross junction boundaries
     #
     # If no error is encountered while tracking, then the project files
     # are rewritten inline.
     #
-    def track(self, targets, *, selection=PipelineSelection.REDIRECT, except_targets=None, cross_junctions=False):
+    def track(self, targets, *, selection=_PipelineSelection.REDIRECT, except_targets=None, cross_junctions=False):
 
         elements = self._load_tracking(
             targets, selection=selection, except_targets=except_targets, cross_junctions=cross_junctions
@@ -389,14 +389,14 @@ class Stream:
     #
     # Args:
     #    targets (list of str): Targets to pull
-    #    selection (PipelineSelection): The selection mode for the specified targets
+    #    selection (_PipelineSelection): The selection mode for the specified targets
     #    ignore_junction_targets (bool): Whether junction targets should be filtered out
     #    remote (str): The URL of a specific remote server to pull from, or None
     #
     # If `remote` specified as None, then regular configuration will be used
     # to determine where to pull artifacts from.
     #
-    def pull(self, targets, *, selection=PipelineSelection.NONE, ignore_junction_targets=False, remote=None):
+    def pull(self, targets, *, selection=_PipelineSelection.NONE, ignore_junction_targets=False, remote=None):
 
         use_config = True
         if remote:
@@ -426,7 +426,7 @@ class Stream:
     #
     # Args:
     #    targets (list of str): Targets to push
-    #    selection (PipelineSelection): The selection mode for the specified targets
+    #    selection (_PipelineSelection): The selection mode for the specified targets
     #    ignore_junction_targets (bool): Whether junction targets should be filtered out
     #    remote (str): The URL of a specific remote server to push to, or None
     #
@@ -437,7 +437,7 @@ class Stream:
     # a pull queue will be created if user context and available remotes allow for
     # attempting to fetch them.
     #
-    def push(self, targets, *, selection=PipelineSelection.NONE, ignore_junction_targets=False, remote=None):
+    def push(self, targets, *, selection=_PipelineSelection.NONE, ignore_junction_targets=False, remote=None):
 
         use_config = True
         if remote:
@@ -509,7 +509,7 @@ class Stream:
     #    target (str): Target to checkout
     #    location (str): Location to checkout the artifact to
     #    force (bool): Whether files can be overwritten if necessary
-    #    selection (PipelineSelection): The selection mode for the specified targets
+    #    selection (_PipelineSelection): The selection mode for the specified targets
     #    integrate (bool): Whether to run integration commands
     #    hardlinks (bool): Whether checking out files hardlinked to
     #                      their artifacts is acceptable
@@ -527,7 +527,7 @@ class Stream:
         *,
         location=None,
         force=False,
-        selection=PipelineSelection.RUN,
+        selection=_PipelineSelection.RUN,
         integrate=True,
         hardlinks=False,
         compression="",
@@ -554,7 +554,12 @@ class Stream:
             self._run()
 
         try:
-            scope = {"run": Scope.RUN, "build": Scope.BUILD, "none": Scope.NONE, "all": Scope.ALL}
+            scope = {
+                _PipelineSelection.RUN: Scope.RUN,
+                _PipelineSelection.BUILD: Scope.BUILD,
+                _PipelineSelection.NONE: Scope.NONE,
+                _PipelineSelection.ALL: Scope.ALL,
+            }
             with target._prepare_sandbox(scope=scope[selection], directory=None, integrate=integrate) as sandbox:
                 # Copy or move the sandbox to the target directory
                 virdir = sandbox.get_virtual_directory()
@@ -612,7 +617,7 @@ class Stream:
     # Args:
     #    targets (str): Targets to show the cached state of
     #
-    def artifact_show(self, targets, *, selection=PipelineSelection.NONE):
+    def artifact_show(self, targets, *, selection=_PipelineSelection.NONE):
         # Obtain list of Element and/or ArtifactElement objects
         target_objects = self.load_selection(targets, selection=selection, use_artifact_config=True, load_refs=True)
 
@@ -639,7 +644,7 @@ class Stream:
     #
     def artifact_log(self, targets):
         # Return list of Element and/or ArtifactElement objects
-        target_objects = self.load_selection(targets, selection=PipelineSelection.NONE, load_refs=True)
+        target_objects = self.load_selection(targets, selection=_PipelineSelection.NONE, load_refs=True)
 
         artifact_logs = {}
         for obj in target_objects:
@@ -667,7 +672,7 @@ class Stream:
     #
     def artifact_list_contents(self, targets):
         # Return list of Element and/or ArtifactElement objects
-        target_objects = self.load_selection(targets, selection=PipelineSelection.NONE, load_refs=True)
+        target_objects = self.load_selection(targets, selection=_PipelineSelection.NONE, load_refs=True)
 
         elements_to_files = {}
         for obj in target_objects:
@@ -689,7 +694,7 @@ class Stream:
     # Args:
     #    targets (str): Targets to remove
     #
-    def artifact_delete(self, targets, *, selection=PipelineSelection.NONE):
+    def artifact_delete(self, targets, *, selection=_PipelineSelection.NONE):
         # Return list of Element and/or ArtifactElement objects
         target_objects = self.load_selection(targets, selection=selection, load_refs=True)
 
@@ -773,7 +778,7 @@ class Stream:
     def workspace_open(self, targets, *, no_checkout, force, custom_dir):
         # This function is a little funny but it is trying to be as atomic as possible.
 
-        elements = self._load(targets, selection=PipelineSelection.REDIRECT)
+        elements = self._load(targets, selection=_PipelineSelection.REDIRECT)
 
         workspaces = self._context.get_workspaces()
 
@@ -910,7 +915,7 @@ class Stream:
     #
     def workspace_reset(self, targets, *, soft):
 
-        elements = self._load(targets, selection=PipelineSelection.REDIRECT)
+        elements = self._load(targets, selection=_PipelineSelection.REDIRECT)
 
         nonexisting = []
         for element in elements:
@@ -1013,7 +1018,7 @@ class Stream:
             else:
                 output_elements.add(e)
         if load_elements:
-            loaded_elements = self._load(load_elements, selection=PipelineSelection.REDIRECT)
+            loaded_elements = self._load(load_elements, selection=_PipelineSelection.REDIRECT)
 
             for e in loaded_elements:
                 output_elements.add(e.name)
@@ -1183,16 +1188,16 @@ class Stream:
     #
     # Args:
     #    targets (list of str): Targets to load
-    #    selection (PipelineSelection): The selection mode for the specified targets
+    #    selection (_PipelineSelection): The selection mode for the specified targets
     #    except_targets (list of str): Specified targets to except
     #    cross_junctions (bool): Whether tracking should cross junction boundaries
     #
     # Returns:
     #    (list of Element): The tracking element selection
     #
-    def _load_tracking(self, targets, *, selection=PipelineSelection.NONE, except_targets=(), cross_junctions=False):
+    def _load_tracking(self, targets, *, selection=_PipelineSelection.NONE, except_targets=(), cross_junctions=False):
         # We never want to use a PLAN selection when tracking elements
-        assert selection != PipelineSelection.PLAN
+        assert selection != _PipelineSelection.PLAN
 
         elements, except_elements, artifacts = self.__load_elements_from_targets(
             targets, except_targets, rewritable=True
@@ -1234,7 +1239,7 @@ class Stream:
     #
     # Args:
     #    targets (list of str): Main targets to load
-    #    selection (PipelineSelection): The selection mode for the specified targets
+    #    selection (_PipelineSelection): The selection mode for the specified targets
     #    except_targets (list of str): Specified targets to except from fetching
     #    ignore_junction_targets (bool): Whether junction targets should be filtered out
     #    use_artifact_config (bool): Whether to initialize artifacts with the config
@@ -1249,7 +1254,7 @@ class Stream:
         self,
         targets,
         *,
-        selection=PipelineSelection.NONE,
+        selection=_PipelineSelection.NONE,
         except_targets=(),
         ignore_junction_targets=False,
         use_artifact_config=False,
@@ -1267,8 +1272,8 @@ class Stream:
             if not load_refs:
                 detail = "\n".join(artifact.get_artifact_name() for artifact in artifacts)
                 raise ArtifactElementError("Cannot perform this operation with artifact refs:", detail=detail)
-            if selection in (PipelineSelection.ALL, PipelineSelection.RUN):
-                raise StreamError("Error: '--deps {}' is not supported for artifact refs".format(selection))
+            if selection in (_PipelineSelection.ALL, _PipelineSelection.RUN):
+                raise StreamError("Error: '--deps {}' is not supported for artifact refs".format(selection.value))
 
         if ignore_junction_targets:
             elements = [e for e in elements if e.get_kind() != "junction"]
@@ -1285,7 +1290,7 @@ class Stream:
         selected = self._pipeline.get_selection(self.targets, selection, silent=False)
         selected = self._pipeline.except_elements(self.targets, selected, except_elements)
 
-        if selection == PipelineSelection.PLAN and dynamic_plan:
+        if selection == _PipelineSelection.PLAN and dynamic_plan:
             # We use a dynamic build plan, only request artifacts of top-level targets,
             # others are requested dynamically as needed.
             # This avoids pulling, fetching, or building unneeded build-only dependencies.
diff --git a/src/buildstream/types.py b/src/buildstream/types.py
index 180044dbd..0e596bddb 100644
--- a/src/buildstream/types.py
+++ b/src/buildstream/types.py
@@ -230,6 +230,36 @@ class _CacheBuildTrees(FastEnum):
     NEVER = "never"
 
 
+# _PipelineSelection()
+#
+# Defines the kind of pipeline selection to make when the pipeline
+# is provided a list of targets, for whichever purpose.
+#
+# These values correspond to the CLI `--deps` arguments for convenience.
+#
+class _PipelineSelection(FastEnum):
+
+    # Select only the target elements in the associated targets
+    NONE = "none"
+
+    # As NONE, but redirect elements that are capable of it
+    REDIRECT = "redirect"
+
+    # Select elements which must be built for the associated targets to be built
+    PLAN = "plan"
+
+    # All dependencies of all targets, including the targets
+    ALL = "all"
+
+    # All direct build dependencies and their recursive runtime dependencies,
+    # excluding the targets
+    BUILD = "build"
+
+    # All direct runtime dependencies and their recursive runtime dependencies,
+    # including the targets
+    RUN = "run"
+
+
 ########################################
 #           Type aliases               #
 ########################################